The invention refers to an apparatus for filtering plasticized thermoplastic synthetic plastic material, comprising a housing with at least one inflow channel for the material to be filtered, which channel leads this material to the inflow side of at least one filter element, whereby the filter element has fine filtering holes for the filtrate and from the outflow side of the filter element to at least one outflow channel for the filtrate leads out of the housing, and whereby at the inflow side of the filter element scrapers moved relative to the filter element are disposed their scraping edges brushing over the filter element; push the waste residues across the surface of the filter element to at least one discharge channel, and whereby, in the direction in which the waste residues are being carried off, there is a non-perforated zone adjoining the zone of the filter element with filtering holes, which non-perforated zone is also brushed over by the scrapers.
The invention also refers to a filter element for an apparatus of this kind.
An apparatus of this kind is known (WO 93/15819). There, the non-perforated zone adjoining the hollow cylindrical perforated filter area is executed as a solid cylinder to increase the resistance to torsional strain of the filter element rotating around its longitudinal axis.
The object of the invention is to improve an apparatus for filtering plasticized thermoplastics synthetic plastic masses of the aforementioned kind by further increasing its service life and filtering efficiency. The invention solves this task by having grooves present in the surface of the non-perforated zone of the filter element facing the scrapers, which grooves are placed in a direction which intersects the direction of the scraping edges scraping over these grooves such that the thickened material on the non-perforated zone is transported to an even greater extent toward the discharge channel.
The invention is based on the following findings: in most of the currently known filter apparatuses, the scrapers glide over the perforated area of the filter, pushing the impurities separated out from the melt being filtered in front of them and toward the discharge channel. This residual material thickens increasingly as it moves toward the discharge channel and the usually solid impurities have an abrasive effect on the filter area. For this reason, there is extreme wear in the filter element in the discharge channel area which leads to the removal of the material of the filter element so that the filter element in this area is ultimately unable to withstand the pressures being exerted against it anymore and breaks. The danger of this happening increases with the pressure exerted by the synthetic plastic material fed in to be filtered, which pressure can reach 300 bar and more in modern filter devices. The wear of the filter element is increased even more by the holes in the filter area acting more or less as abutments for the residue material being transported by the scrapers, because granular constituents in the residue material fit positively into these holes and enter them.
Moreover, investigations have also shown that the concentration of solid constituents in the material the scrapers push along in front of them is extremely high right in front of the scrapers, i.e. this material is thickened to a high degree. However, immediately behind the scrapers, a clean zone emerges into which zone the pressurized material being filtered flows. When such still substantially unfiltered constituents of the melt enter the area of the discharge channel, relatively pure material is drawn off there. The material conveyed off in the discharge channel, usually by means of a discharge screw, is therefore highly non-homogeneous in its composition, because there is a constant changeup between portions coming from the areas behind the scrapers with portions coming from in front of it. Those portions from in front of the scrapers usually contain highly thickened material with a relatively high proportion of impurities whereas those from behind the scrapers are relatively high in plastics and the share of impurities is quite low.
By means of the measure taken in accordance with the invention, the scrapers now do not scrape across an area of the filter element perforated throughout but rather the non-perforated zone, with its grooves, adjoining the discharge channel create what are more or less abutments for the circulating solid material so that this material is transported to a greater extent toward the waste discharge channel. As to say, the grooves, in effect, promote conveyance. Thereby in an advantageous manner the effect is achieved that the impurities collect in the area adjoining the waste discharge channel, pushing away the plastics material, that is the useful constituents of the melt, so that the useful constituents once again arrive in the perforated area of the filter and are filtered through the holes there. As a result, the material found in the discharge channel becomes more homogenous than that in the known design initially describedxe2x80x94and also relative to other common types of filters whose filter surface is brushed over by scrapersxe2x80x94and, surprisingly, less plastic that is lost, reaches the discharge channel, than was expected. One would expect, namely, that the non-perforated zone would result in a worse filtration of the material, however, in reality just the contrary is the case.
Moreover, the grinding effect becomes less on the impurity constituents of the thickened mass transported by the scrapers in comparison to a perforated zone of the filter since the grooves in the design in accordance with the invention have a less anchoring effect on the solid constituents of the residual material than the holes do.
The invention is applicable both to filter elements which are longitudinally symmetrical, that is more or less hollow cylinders or filter elements shaped like the envelopes of a cone, e.g. such as those in accordance with the aforementioned WO 93/15819, as well as, to particular advantage, to disk-shaped filter elements, particularly those shaped as circular disks, in which the discharge channel for the contaminants is positioned centrally. The reason for this latter feature is that the thickening of the residual material becomes especially great there. In any case, the filter element can be impinged either from within or from without by the material being filtered, i.e. the material can flow through the filter element either from in to out or from out to in.
The invention is likewise non dependent on whether it is the filter element which is moved (as with the aforementioned WO) or the scrapers or both elements.
In accordance with a preferred embodiment of the invention, the non-perforated zone of the filter element has a greater wall thickness than the perforated zone. The advantage of this approach is that the grooves in the non-perforated zone can be deep enough to be able to fulfill their desired conveyance function to a great extent. In a hollow cylindrical filter element, as the document mentioned initially indicates, the non-perforated zone does not by any means have to be solidly executed, in fact, it can be in the shape of a hollow cylinder, just as the perforated filter zone is. The hollow space created thereby not only reduces the weight and the material required, it can also be used to accomodate other parts of the apparatus.
The ratio of the length of the non-perforated zone, as measured in the direction in which the residues are removed, to that of the perforated zone measured in the same direction is advisably from 0.5:1 to 1:1, this ratio will depend on the prevailing circumstances and the type of synthetic plastic material usually being processed.
In accordance with a preferred embodiment of the invention, the axes of the holes in the perforated zone are inclined in the direction of the removal of the residues, preferably at an angle of 60 to 80xc2x0 to the surface of the filter element. When the filter element is disk-shaped, it is advisable within the scope of the invention to have the plastics material be fed in from the edge, whereby the discharge channel is arranged in the center of the filter element and a screw is positioned in the discharge channel to transport the residues. In a disk-shaped filter element of this kind, it is also favorable within the scope of the invention to have the axes of the filtering holes in the perforated zone be inclined relative to the radial direction of the filter element, preferably at an angle of 30 to 90xc2x0. In such a case, the axes of the filtering holes are thus inclined both in relation to the radial direction of the filter element and to the plane of the filter element However, in no instance may the holes be inclined opposite the motion direction of the scraper edges, because this would cause the grains contained in the contaminant material to be pressed into the filtering holes immediately. This, on the one hand would block these filtering holes and, on the other, exert an extreme strain on the shear edges of the scrapers.
The grooves, like the scraping edges of the scrapers, need not be straight. In fact, in many cases it is favorable to have the scrapers and/or grooves run along curved lines, e.g. helix-like. In this case, it is advisable within the scope of the invention to have the grooves curved opposite to the curve of the scrapers.
Suitably, the grooves have a saw-like cross section whose steeper flank is disposed on the back side of the scrapers. Here, the grooves create a positive interlocking effect between the thickened waste residues and the scraper edges. This steeper flank forms an edge which blocks the thickened residue material, thus promoting the transport of this material in the direction towards the discharge channel.
The scrapers can be made of continuous strips. However, it is more favorable within the scope of the invention to arrange the scrapers along strip-like paths, whereby however each path is divided into spaced scraper elements, whereby the gaps between adjacent scraper elements of successive scrapers are offset in relation to each other, as seen in the direction of rotation of the scrapers or of the filter element. The advantage of subdividing the scraper strips into individual elements is that the scraper edges formed by these scraper elements are sure to engage the surface of the filter element even if the filter element bends from the high pressure exerted by the material being filtered, which pressure can reach 300 bar and more. This staggered arrangement of the scraper elements ensures that portions of the material passing through the gaps between adjacent scraper elements of a preceding scraper strip are caught by the elements of the following scraper strip. For safety""s sake, adequate overlap should be provided for in each case, otherwise, ring-shaped zones of contaminants would build up which would no longer be able to be removed. It would of course be possible to divide the scraper strip into elements that gaplessly adjoin each other, however solid particles, e.g. glass shards, could then adhere to the transition point between two adjacent scraper elements, which particles would be unable to free themselves from this point while the scraper strip rotates and would merely be transported in a circle. The result would be circular pattern of wear of the filter element. However, if the individual scraper elements belonging to a given path are spaced apart from each other, the solid particles can pass through the gaps between the two scraper elements and then be caught by the following scraper element and be transported to the waste discharge channel, at the latest due to the combined effect of the latter element and the grooves in the non-perforated area of the filter element.
A very high degree of efficiency is achieved with a design in which the inflow channel is placed between two parallel disk-shaped filter elements which are brushed over by scrapers attached to a scraper carrier, preferably disk-shaped too, arranged between the filter elements and associated to both filter elements. A construction of this kind can be executed with a single discharge screw in the discharge channel, however the disadvantage of this arrangement is that the residues can flow directly to the screw from just one of the two filter disks. From the other filter disk, the residues would have to squeeze through openings in the scraper carrier which openings are separated from each other by spoke-like webs. With this, it is difficult to reroute the residues consisting mostly of solids and by such a design it is impossible to achieve high degrees of thickening. It is therefore more favorable within the scope of the invention to assign each of the two filter elements to its own screw section, whereby the two screw sections convey the material in opposite directions. In this way, the aforementioned rerouting of the residues is avoided. If a temperature control is assigned to at least one of the two screw sections, the conditions in the discharge material can be influenced by regulating the temperature of the plastics material in the area of the waste discharge screw. If one determines, for example, that the waste discharged contains 10% more plastics material on one screw side, then the temperature control can be used on this side to thicken the material being removed such that the portion of plastics material in the waste discharged on this side is reduced. In this way, the discharges on both sides can be equalized.
A further important aspect is that a provision must be made so the batches of solids transported by the scrapers to the discharge channel are reliably engaged by the screw bearingly supported in the discharge channel. The ideal situation would be if the screw flights were simply a continuation of the scraper strips. However, this is difficult to achieve in practice since provisions are usually made for a large number of scraper strips and having the same number of screw flights would mean the screw flights are very close together. This, in turn, would make it more difficult for material to be taken along by the screw. It is therefore preferable to arrange the system such that the material transported to the periphery of the screw by the scraper strips always strikes a gap between two adjacent screw flights. The number of screw flights in this case is therefore at most equal to the number of scraper strips and usually lower.
In the perforated filter area, it is advisable for the scraper elements to have a resilient support and to therefore be pressed against the filter cylinder or filter disk. This pressure can be exerted by spring members, but also, additionally or alternatively, by the pressure of the supplied material or by the difference in pressure between the inflow and outflow side of the filter. This difference in pressure can be approx. 100 bar. By contrast, in the non-perforated zone of the filter cylinder or filter disk, the scrapers can be rigid elements arranged on the scraper carrier, that is, for example, a shallow-flight screw rigidly mounted on a scraper carrier disk.
A filter element in accordance with the invention for an apparatus of the described type in accordance with the invention is based on a design in which a non-perforated zone adjoins a perforated zone. A filter element of this kind is characterized by the fact that a provision is made to have grooves made in the non-perforated zone on the inflow side of the filter element. Preferably, the wall in the non-perforated zone is thicker than the one in the perforated zone.